This invention relates to a phase locked loop circuit. Specifically, the present invention relates to a sweep acquisition phase locked loop wherein an oscillator's frequency varies over a relatively wide range of frequencies to increase loop capture range. More specifically, the present invention relates to a sweep acquisition phase locked loop wherein sweep rate of a tracking signal is substantially greater than is permitted by loop bandwidth.
As is well known in the art, phase locked loops (PLLs) can lock to a carrier which has a frequency that is not precisely known. The ability of the PLL to lock to the carrier depends upon the PLL's bandwidth and the signal-to-noise ratio of the carrier. If the carrier is known to exhibit a frequency within a small range of frequencies, then the PLL need not employ sweep acquisition because the natural hunting tendency of the PLL will permit the loop to lock to the carrier.
However, if the frequency uncertainty of the carrier is relatively large, then the PLL may employ a swept acquisition. Swept acquisition refers to the use of additional circuitry which causes an oscillator portion of the PLL to vary its oscillation frequency over a relatively large range of frequencies. However, an undesirable consequence of sweep acquisition is that the carrier must exhibit a relatively large signal-to-noise ratio before lock can be achieved.
The term "sweep rate " refers to the speed at which the oscillator's frequency changes. In PLLs which utilize swept acquisition, slower sweep rates permit locking to carriers having lower signal-to-noise ratios.
A problem associated with tracking PLLs occurs when carrier frequency may occur anywhere within a relatively large range of frequencies and when the carrier may exhibit a low signal-to-noise ratio. In order to lock to a signal having a low signal-to-noise ratio, the PLL sweep rate must be fairly slow. Further, the sweep must vary the frequency of the oscillator over a relatively large frequency range to insure acquisition of the carrier. Accordingly, such PLL's may require an undesirably long period of time to achieve a locked condition after the application of a carrier.